Currently, physicians use a variety of visualization methods for 3D data: Volume Rendering, Multi-Planar Reformat, Maximum Intensity Projection, Surface Shaded display to name some of the most popular methods.
Visualization and segmentation are tightly coupled in the postprocessing workflow. The earliest and most prevalent visualization technique for MR \& CT angio studies has been the maximum intensity projection (MIP). MIP visualization projects the entire volume along a single direction vector onto a plane. The intensity value of a pixel in the projected image is the maximum intensity value of all voxels which project to that particular 2D point. MIP is an excellent tool for vascular visualization, but it lacks depth information. The impression of 3D is conveyed through interactive rotation of the view, but it is difficult for the user to discern when vessels pass in front or behind one another. It is also difficult to see surface features of vessels (for example the small bump of a nascent aneurysm), because surface information is lost in the projection.
Surface Shaded Display (SSD) is a technique in which the volume is initially thresholded so that all pixels above a certain intensity value are labeled as belonging to the object(s) to be visualized. Object voxels are considered to be completely opacified and are rendered using lighting and local surface normal computations to reveal details through shading and specular highlights. The highlights provide strong visual clues to surface irregularities like bumps and depressions. Because only those voxels on the surface of the object are visible to the user, SSD can be easily optimized to operate at real-time speed without the need for special hardware acceleration. SSD is very effective for visualizing CT vascular data which can be reliably thresholded. For MR, however, inhomogeneities in the signal due to coil proximity artifacts and gain irregularities make SSD unreliable in moderate to low quality MR studies.
Multi-Planar Reformat (MPR) is a known technique in which a 2-D image is created by resampling the volume on an arbitrary plane. It produces an image which looks like an original slice, but with an arbitrary position and orientation. Since traditional MPR is limited to a plane, it is often not possible to visualize curved structures in a single MPR view.
Also known in the art is curved MPR, a technique which resamples 3D data into a 2D image on a surface created by sweeping a planar curve along a linear path orthogonal to the plane containing the curve. Curved MPR is better than traditional MPR at capturing curved structures in a single reformat, but it cannot track curvilinear structures with highly tortuous trajectories.